1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) apparatus for an internal combustion engine, and more particularly relates to an apparatus wherein, even if a solenoid valve causes a failure in an open condition thereof, exhaust gas recirculation is automatically cut at an idling condition of the engine to thereby prevent engine stall at the idling condition.
2. Description of the Related Art
An EGR apparatus is provided to an internal combustion engine to suppress exhaust of nitrogen oxides to the atmosphere. FIG. 3 illustrates an EGR apparatus for an internal combustion engine which has a first turbocharger 203 and a second turbocharger 204. This apparatus is the subject of Japanese Patent Application HEI 3-27461, filed Feb. 21, 1991, by the present applicant. In the apparatus, a pressure in the diaphragm chamber 205b of the EGR valve 205 is controlled by the duty-control three-way vacuum switching valve (VSV) 207. Further, in order to obtain a fail-safe design of the three-way VSV 207 in the event of a failure at a full-open condition thereof, that is, in order to cut the EGR even if the VSV 207 causes a failure at an idling condition of the engine, a vacuum control valve (VCV) 208 is installed in a passage between the VSV 207 and the EGR valve 205.
More particularly, when the VSV operates normally, the EGR apparatus operates as follows:
At an idling condition of the engine where a throttle valve 201 is fully closed and a duty ratio value of the VSV 207 is 0%, a negative pressure generated in a surge tank 202 and transmitted through a port 202a is checked at the VSV 207 and is not transmitted to the EGR valve 205. The negative pressure in the surge tank 202 is transmitted through the port 202b to the VCV 208 to raise a diaphragm of the VCV to open an atmosphere communication port 208a. As a result, the atmospheric pressure from the intake conduit 212 is introduced into the diaphragm chamber 205b of the EGR valve 205 to close the EGR valve 205 and to cut the EGR.
At a low engine load condition in which the throttle valve 201 is partially open and the duty ratio value of the VSV 207 is about 30% (also partially open), the negative pressure of the surge tank 202 is transmitted to the EGR valve 205 through the VSV 207, an EGR valve modulator 206, and the VCV 208. In this instance, the port 202b is located upstream of the partially opened throttle valve and the atmospheric pressure is transmitted from the port 202b to the diaphragm chamber of the VCV 208 in order to close the atmosphere communication port 208a of the VCV 208. As a result, the negative pressure in the diaphragm chamber 205b of the EGR valve 205 is maintained, and the EGR valve 205 is open so that the EGR is conducted.
At a high engine load condition in which the throttle valve 201 is fully open and the duty ratio value of the VSV 207 is substantially 100% (fully open), the pressure in the surge tank 202 is approximately equal to or greater than the atmospheric pressure. The VSV 207 cuts communication of the EGR valve 205 with the surge tank 202 and causes the EGR valve 205 to communicate with the intake conduit 212, so that the atmospheric pressure from the intake conduit 212 is transmitted through the VSV 207 to the EGR valve 205 to close the EGR valve 205 and to cut the EGR.
When the VSV 207 causes a failure in the fully open condition thereof and the throttle valve 201 is fully closed, the negative pressure in the surge tank 202 is transmitted to the EGR valve 205 and may cause an engine stall, if the VCV 208 were not provided. However, at the fully close condition of the throttle valve 201, the negative pressure generated in the surge tank 202 is introduced, through the port 202b, into the diaphragm chamber of the VCV 208 to open the atmosphere communication port 208a of the VCV 208. An atmospheric pressure is introduced through the port 208a into the diaphragm chamber 205b of the EGR valve 205 to close the EGR valve 205. In this way, even if the VSV causes a failure, the EGR is cut at an engine idling condition to prevent an engine stall.
However, there are the following problems with the above-described EGR apparatus:
One problem is that when both the VSV 207 and the VCV 208 cause failures at the same time, that is, when the VSV 207 causes a failure at a fully open condition thereof and the VCV 208 causes a failure at a fully closed condition of the atmosphere communication port thereof, the EGR cannot be cut at an engine idling condition.
Another problem is that cutting of the EGR tends to be delayed because the VCV 208 installed in the passage increases the volume of the passage connecting the surge tank and the EGR valve, thus delaying the response, though the EGR should be cut swiftly when the throttle valve is closed.